1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method and, in particular, to an image processing apparatus, an image processing method and a computer-readable medium storing a program for reducing color bleeding.
2. Description of the Related Art
A digital video camera and a digital camera available recently on the market have used image sensors with a large number of pixels to provide a high picture-quality image.
Color bleeding, which occurs by the influence of chromatic aberration resulting from a difference in an image forming position for each optical wavelength for the reasons of the miniaturization of a pixel and the use of a miniature lens, tends to appear on an image. Furthermore, the color bleeding appears due to the generation of a false color signal in interpolation because color filters with a plurality of colors provided correspondingly with the pixels of a solid-state image sensor are arranged in a mosaic form.
A method for reducing the color bleeding in a captured image has been proposed. Japanese Patent Application Laid-Open No. 2000-076428 discuses a technique, which identifies a lens used for capturing an image, reads aberration information, produces a correction parameter, and moves coordinates of a color signal to be corrected according to the correction parameter.
A color bleeding characteristic, however, complicatedly varies with an image-height position from the center of an optical axis to a target pixel, a zoom lens position, the opening diameter of an iris, and a focus-lens position. In a configuration for reading the aberration information of a lens as discussed in Japanese Patent Application Laid-Open No. 2000-076428, aberration information needs to be stored for each image-height position, zoom lens position, opening diameter of an iris, focus-lens position, and kind of a lens.
For this reason, a memory with a large capacity is required to store the aberration information. Since the area where the color bleeding is caused by interpolating a color signal output from an image sensor on which color filters with a plurality of colors are arranged in a mosaic form depends on an object to be captured, the amount of movement in coordinates of a color signal cannot be identified in advance.
There has been proposed a technique for extracting an area where the color bleeding seems to be caused from an image instead of reading the previously stored aberration information of a lens, and reduces the color bleeding in the area. Japanese Patent Application Laid-Open No. 2003-102027 discusses a technique which detects a difference between the rates of change in two color components from an image, determines an area greater in the difference as an area where the color bleeding is caused, and reduces the color bleeding.
However, it is difficult to completely eliminate an erroneous determination if an area where the color bleeding is caused is determined from an image without reading the stored aberration information of a lens.
In a configuration in which a difference between the rates of change in two color components is detected, as discussed in Japanese Patent Application Laid-Open No. 2003-102027, if an object itself has a pattern satisfying such a condition, while the color bleeding is not caused, the object can be determined as a color-bleeding area. For this reason, even if color bleeding is not caused in an area, the density of color in the area is lowered or signals of the area may be replaced with signals of pixels surrounding the area.
This causes a problem that the object affects the determination result even in a configuration of determining an area where the color bleeding is caused, and an extent of the color bleeding from the luminance component and the color component of an image other than in the configuration discussed in Japanese Patent Application Laid-Open No. 2003-102027.
Strict conditions for determining a color bleeding area need to be set to avoid such a problem. However, simply setting the strict conditions for determining a color bleeding area increases areas, which are not determined as a color bleeding area irrespective of areas where the color bleeding is caused, which does not contribute enough to improve picture quality.
When an image subjected to a process for reducing the color bleeding is subjected to an expansion process, both of the area where the density of color is reduced due to erroneous determination even though the color bleeding is not caused and the area where the density of color is not reduced even though the color bleeding is caused, are expanded. The area where the density of color is reduced even though the color bleeding is not caused is probably more conspicuous by its unnaturalness than the area where the color bleeding remains.
This may occur when an image is subjected to a compression process. In the area where the density of color is reduced even though the color bleeding is not caused, the lower the compression rate of an image, the clearer and the more conspicuous the unnatural area of the image.
In view of the above described problems, there is a need for an image processing apparatus and an image processing method capable of effectively correcting the color bleeding without the color bleeding being made conspicuous even if an erroneous correction is performed.